Antennas have been proposed which use the theory of operation of quarter or half wavelength antenna in combination with a vehicle window having a thin IR reflective film or conductive coating on or between the layers of the glass window. For example, U.S. Pat. Nos. 4,849,766, 4,768,037, and 4,864,316 illustrate a variety of antenna shapes that are formed by a thin film on a vehicle window. U.S. Pat. No. 5,670,966 discloses an automotive antenna having several electrically interconnected coating regions. U.S. Pat. Nos. 5,083,135 and 5,528,314 illustrate a vehicle antenna having a transparent coating in the shape of a “T”. U.S. Pat. No. 6,448,935 discloses an antenna having a two-piece conductive coating that is used as AM and FM antenna that are separated to reduce AM noise and improve system performance.
Other designs include a slot antenna that is formed between the metal frame of a window and a conductive transparent film or coating that is bonded to the window wherein an outer peripheral edge of the transparent film is spaced from the inner edge of the window frame to define a slot antenna. Such antennas are illustrated in U.S. Pat. Nos. 4,707,700 and 5,355,144. U.S. Pat. No. 5,898,407 purports to improve transmission and reception of radio frequency waves by use of a conductive coating with at least one edge that overlaps the window frame of the vehicle body to establish a short to ground by coupling for high frequency signals. U.S. Pat. No. 7,764,239 B2 discloses the use of a laser beam to create a slot antenna by removing the conductive coating. Since the antenna feeding cable has to cross the slot, a large space on the window is required to conceal the antenna feed structure, thus restricting the antenna location to top of the window. U.S. Pat. No. 6,320,276, B1 discloses an antenna feeding structure that uses a capacitive coupling apparatus in which wires are capacitively coupled to the slot antenna.
From an aesthetic point of view, a slot antenna is generally preferred because the antenna is invisible so that it has broader application. Another advantage of slot antennas is heat load reduction because the slot antenna involves removal of an area of the heat reflective coating that is relatively small compared to many other antenna designs. However, slot antennas also present several technical challenges, especially when used in connection with the vehicle windshield window. First, the area around the window perimeter for locating the antenna elements is limited. That limitation makes it difficult to design an antenna that meets typical performance requirements. Secondly, the size and dimensions of the slot antenna lend window slot antennas more to use with the VHF frequency band. At the UHF band, the slot antenna generally has a much weaker resonance and gain because the UHF band is carried in the higher order modes of the slot for which impedance is much higher and impedance matching of the antenna more difficult. For example, the perimeter of the window defines the maximum slot length. Maximum slot length determines the fundamental mode and the lowest frequency for the antenna. Usually that frequency is in the VHF band. Typical windshield and back glass window slot antennas can cover the FM frequency band, but not the TV VHF and UHF bands (47 MHz-860 MHz).
Therefore, it would be advantageous to provide an antenna, particularly a windshield antenna, that is hidden and that also supports a wide frequency band for different applications.